The present invention relates to a wet-type photocell which is capable of converting light energy to electric energy by making use of a dye sensitizing effect and also by introducing light photons into the interior of an anode to reflect therein the thus-introduced light photons a plurality of times. The present invention particularly relates to an internally reflective, dye sensitized, wet-type photocell suitably adaptable as a photosensitive element or a solar cell.
Dry-type photocells are well-known. These include, for example, so-called solar cells, in which a p-n junction is formed in the vicinity of the surface of a semiconductive crystal consisting mainly of silicon. However, such conventional solar cells have the drawback that the semiconductive substance used must be of high purity. As a corollary to this, such solar cells are expensive to manufacture and their application is thus restricted to special fields. They are accordingly not suitable as general energy conversion elements.
To compensate for such drawbacks of the above-described conventional dry-type solar cells, there has recently been considered the use of a wet-type photocell which functions sufficiently well even if an inexpensive semiconductor such as a sintered semiconductive material is employed.
However, in such wet-type photocells, which are the objects of such recent consideration, an anode made of any convenient semiconductive substance is subject to dissolution as a result of a photoelectrode reaction, thereby causing the problem that the life of the electrode material is reduced.
Of course, there are some exceptional examples of semiconductive electrodes which are almost free from such a dissolution problem. For example, there are known as such exceptional semiconductive substances tin oxide, titanium dioxide, strontium titanate and the like. All of these semiconductive substances, however, are responsive only to light in the ultraviolet region, and they therefore are accompanied by the problem that they are not adequate and suitable as a conversion element for solar photon energy which is extensively distributed in the visible and near-infrared regions.
For the reasons described above, some attempts have been made with a view to expanding to the longer wavelength side the responsive wavelength region of a stable semiconductive substance which is responsive only to ultraviolet rays and is free from the dissolution problem due to photoelectrode reactions. Each of such attempts makes use of the dye sensitizing effect. In other words, it attempts to convert the energy of photons of a wavelength which is too long to be absorbed by such a semiconductive substance, to electric energy by adding to an electrolyte solution a dye which absorbs photons of a wavelength longer than the characteristic absorption wavelength region of a semiconductive electrode.
In a wet-type photocell containing a dye sensitizer added to the electrolyte solution thereof, it has been found that the energy conversion efficiency of the energy of photons having a wavelength longer than the characteristic absorption wavelength of a semiconductive substance itself to electric energy is extremely low, as indicated clearly from the measurements of photo electromotive forces by using spectral light. Thus, it has been concluded that such wet-type photocells are not suited for practical applications.
As a cause for such a low conversion efficiency, it is known that dye which is dissolved in an electrolyte solution does not take part in the dye sensitizing; only dye which is adsorbed at the interface between the electrode and electrolyte solution contributes to the dye sensitizing effect. However, there has still not been proposed any method or means to solve the problem of low conversion efficiency.